It is well known that a semiconductor device having a pnpn structure is shifted between the two states of the low impedance and the high impedance with respect to an internal impedance. Such a pnpn semiconductor device is described on pages 679 to 681 of "Appl. Phys. Lett. 52(9), 29 February 1988". This pnpn semiconductor device comprises p- and n-outside semiconductor layers corresponding to anode and cathode layers, and p- and n-inside semiconductor layers sandwiched by the p- and n-outside semiconductor layers, wherein bandgap energies of the p- and n-outside semiconductor layers are larger than those of the p- and n-inside semiconductor layers, so that light is emitted with a high efficiency from the pnpn semiconductor device in the low impedance state.
This kind of a pnpn semiconductor device is applied to an optical memory, an optical logic circuit, etc. For this purpose, a voltage slightly lower than a switching voltage is applied to the pnpn semiconductor device which is then triggered by an input light, so that the pnpn semiconductor device is shifted from the high impedance state to the low impedance state to emit an output light. This is a stage at which information is written. This low impedance state is held in the pnpn semiconductor device, to which a voltage slightly higher than a holding voltage is applied. This is a stage at which the written information is held. Then, a read out voltage slightly lower than the switching voltage is applied to the pnpn semiconductor device to emit an output light. This is a stage at which the written information is read out. The information is erased by applying a voltage slightly lower than zero voltage to the pnpn semiconductor device. This is a stage at which the written information is erased.
However, the above described method for driving a pnpn semiconductor device has a disadvantage in that a light energy which is required for shifting the pnpn semiconductor device to the low impedance state is considerably large. In the pnpn semiconductor device as described in the aforementioned Appl. Phys., the required light energy is 1pJ. Consequently, a light energy of 100 .mu. W is necessary to turn the pnpn semiconductor device on by a switching speed of 10nS (equivalent to 100 Mb/s). This value of the light energy is a considerably large value in view of an intensity of light emitted from light emitting diodes and semiconductor lasers presently available.